1. Field of the Invention
The present invention relates to a liquid crystal display device for use in a television set, a personal computer, a word processor, an OA (office automation) apparatus or the like, and a method for driving the same.
2. Description of the Related Art
A liquid crystal (hereinafter "LC") display device for use in a television set, a personal computer, a wordprocessor, an OA (office automation) apparatus or the like functions by utilizing the refraction anisotropy of LC molecules. Specifically, a voltage is applied to a LC layer so as to generate an electric field for causing optical modulation.
In one method for applying a voltage to the LC layer, gate lines and source lines are arranged in a matrix shape, and a pixel electrode and a thin film transistor (hereinafter referred to as "TFT") are formed at each region surrounded by a pair of gate lines and a pair of source lines. Thus, the TFT controls the voltage to be applied to the pixel electrode.
A typical example of a conventional LC display device based on the above-mentioned principle is illustrated in FIGS. 18 to 20. FIG. 18 is a cross-sectional view of the LC display device; FIG. 19 is a plan view of a matrix substrate used for the LC display device; and FIG. 20 is a cross-sectional view taken at line XX-XX' in FIG. 19.
As shown in FIG. 18, this LC display device includes a LC layer 103 filled in an interspace between a matrix substrate 101 and a counter substrate 102 (implemented as a pair of light-transmitting substrates, e.g., glass), with spherical spacers (not shown) dispersed therein.
As shown in FIG. 19, a matrix of source lines 104 and gate lines 105, TFTs 106, and pixel electrodes 107 are provided on the matrix substrate 101. To each pixel electrode 107, a voltage is applied from a source line 104 via a corresponding TFT 106. On the counter substrate 102, a light-shielding film (not shown) having an aperture corresponding to the pixel electrode, a color filter (not shown), and a counter electrode (not shown) having a planar shape are provided.
FIG. 20 shows a cross section of the TFT 106. A semiconductor layer 110 is provided on a gate electrode 108 which branches out from the gate line 104 (FIG. 19) with an insulative layer 109 interposed therebetween. A source electrode 111 and a drain electrode 112 are formed on the semiconductor layer 110.
The above-described LC display device utilizes the dielectric constant anisotropy of LC molecules.
On the other hand, a LC display device has been proposed (Japanese Laid-open Publication No. 7-64118) which utilizes a magnetic field by taking advantage of the anisotropy of magnetic susceptibility of LC molecules.
As shown in FIG. 21, this LC display device includes a LC layer 115 interposed between a pair of substrates 114, with ferromagnetic elements 113 disposed on one of the substrates 114. The LC 115 present in each region interposed between ferromagnetic elements 113 is controlled by varying the magnetization of the ferromagnetic elements 113 with an external magnetic field application means 116.
In general, the density of magnetic energy fm of LC molecules in a magnetic field can be expressed by eq. 1: EQU fm=-1/2x.perp.H.sup.2 -1/2.DELTA.x(n.multidot.H).sup.2 eq. 1
where
.DELTA.x=x.parallel.-x.perp.: anisotropy of magnetic susceptibility; PA1 x.parallel., x.perp.: magnetic susceptibility; and PA1 n: orientation of LC molecules.
By applying a magnetic field to LC molecules which have a positive anisotropy of magnetic susceptibility .DELTA.x, a net magnetic moment of the LC molecules will align the LC molecules in parallel to the direction of magnetic field, hence minimizing the magnetic energy. Thus, the orientation of the LC molecules can be controlled by means of a magnetic field as well as an electric field.
In a conventional LC display device utilizing an electric field, the TFTS 106 as shown in FIG. 20 are formed for recording signal voltages corresponding to the respective electrodes. This TFT 106 acquires stable characteristics by highly accurately superposing respective patterns corresponding to the gate electrode 108, the semiconductor layer 110, the source electrodes 111, and the drain electrodes 112 upon one another. Specifically, the current value of the TFT 106 is substantially in inverse proportion with the distance between the drain electrode 112 and the source electrode 111, and parasitic capacitance is created which is substantially in proportion with the overlap between the gate electrode 108 and the source electrode 111 and/or the gate electrode 108 and the drain electrode 112. The current value and the parasitic capacitance determine the potential of the pixel.
Usually, the distance between the drain electrode 112 and the source electrode 111 is prescribed to be about 10 .mu.m, and the overlap between the gate electrode 108 and the drain electrode 112 and/or the gate electrode 108 and the source electrode 111 is prescribed to be about 1 to 2 .mu.m. Thus, it is necessary to control the final width of each pattern, and the overlap width between patterns, to be 1 .mu.m or less.
In an effort to achieve such high-accuracy patterning, an exposition step is performed by, for example, employing a high precision photolithography technique which utilize an exposure apparatus including a projection lens system and a stage having a high alignment accuracy; herein, the goal is to manufacture devices under engineering conditions with a stringency of about 1 .mu.m.
Moreover, an amorphous silicon (a-Si) is used for the semiconductor layer to form TFTs. The formation of an a-Si layer requires the use of a PE-CVD apparatus for improved layer quality.
The LC display device obtained through the aforementioned techniques provides excellent display quality in exchange for the high cost and low processing ability of the apparatuses used for manufacture (due to the use of a high precision photolithography process and a PE-CVD process). Also, strict process management is required.
Furthermore, this electric field-based conventional LC display device retains electric charges (associated with image signals) by utilizing the LC layer as a capacitor; therefore, the LC layer is required to have a high resistivity. For example, such high resistivity is needed since while driving the LC display device at a high temperature, e.g., 70.degree. C., impurity ions may be generated within the LC layer, thereby lowering the resistivity and causing display unevenness. This may result in a reduced yield of LC display devices.
Thus, conventional LC display devices utilizing an electric field have a large problem in reducing the manufacturing cost and improving the product yield. For these reasons, LC display devices are perceived as expensive products as compared to other image display devices, e.g., CRTs, despite their thin construction, light weight, and high display quality. The high prices of LC display devices are an obstacle to gaining prevalence. Therefore, it is desired to develop a LC display device which can be produced through an easier process.
As for LC display devices utilizing a magnetic field, Japanese Laid-open Publication No. 7-64118 merely explains the principle of achieving optical modulation in LC by using a magnetic field generated by a magnetic body, but fails to disclose any structure of an actual driving means. In other words, the above literature does not describe a method for driving a matrix of pixels arranged as an image display device for use in a television set, a personal computer, a word processor, an OA apparatus, or the like. Therefore, such LC display devices are yet to replace conventional LC display devices.